1. Field of the Invention
The present invention relates to a radiation imaging apparatus, a method of controlling such a radiation imaging apparatus, and a radiation imaging system.
2. Description of the Related Art
In recent years, with development of the semiconductor technology, there are put into practice and are popularized digital X-ray imaging apparatuses using a two-dimensionally arranged sensor in which conversion elements for converting light into an electric signal are formed on a glass substrate.
In U.S. Pat. No. 6,453,008, there is described a radiation detector including plural offset memories having offset data which have been acquired in advance in correspondence with plural modes (sequential read-out, pixel addition, trimming, high frame rate and high irradiation), and which selectively uses an appropriate one of the offset memories in accordance with the corresponding mode.
Moreover, in Japanese Patent Application Laid-Open No. 2004-194702, there is described a digital radiation imaging apparatus adapted for collecting offset data in which a change of scanning mode (fluoroscopy, photographing) is performed in advance.
In fluoroscopy (moving picture radiographing), the frame rate, i.e., the interval between a series of operations for irradiating an object with X-rays to acquire radiation image data is switched for three reasons, described below.
As the first reason, there is mentioned a desired reduction in dosage of exposure to radiation. In recent years, where there is high interest relating to X-ray exposure, and an X-ray diagnosis apparatus or a radiographing technique which can perform radiographing with as low a dosage of exposure to radiation as possible is required, there has been employed an approach to reduce the dosage of exposure to radiation as low as possible by pulse fluoroscopy and setting of a fluoroscopic frame rate. Pulse fluoroscopy is a fluoroscopic radiographing method of irradiating the object with X-rays in a pulse form in correspondence with a set frame rate. The frame rate which has been set in this pulse fluoroscopy is simply X-ray irradiation quantity per unit time. In the case of pulse fluoroscopy, a worker can perform radiographing with the frame rate lowered to a level such that there is no inconvenience in operation, while lowering the dosage of exposure to radiation of the patient. For example, there is mentioned an example where, in a series of works of acquisition of mask image, contrast medium injection start by injector and contrast medium injection end in angiography, frame rate is suitably switched in accordance with working state to suppress dosage of exposure to radiation of a patient as low as possible. Thus, there is required a fluoroscopic system capable of arbitrarily switching frame rate.
The second reason is synchronization with movement of an object. Applications to perform radiographing in synchronism with various movements of the object such as heartbeat or movement of a patient's bed have been studied and developed.
The third reason is that the fluoroscopic system is being applied to various X-ray diagnosis systems. One application is simple CT-like application. CT (Computed Tomography) is a technique for circumferentially scanning the periphery of an object by X-rays to obtain an X-ray tomographic image of an object. Recently, CT has been added, as an optional function of a fluoroscopic system using a flat plane X-ray detector as a detector. In the fluoroscopic system, since an X-ray source and a detector are oppositely disposed, rotation is performed around an object so that radiographing similar to CT can be performed. Since synchronization between angular velocity of rotation and radiographing frame is essential in order to obtain a precise CT image, it is necessary to acquire an image by taking change of angular velocity from start of movement into consideration. For this reason, it is required to be able to make arbitrary changes in frame rate.
As described above, there is the problem that while there is a requirement of switching the frame rate in the fluoroscopic system, offset correction is not sufficient immediately after switching of frame rate in the flat plane X-ray detector as mentioned in the previously cited patent documents. Offset correction is to correct an output which is not dependent upon X-ray quantity which is output from the flat plane X-ray detector. As element of offset, there are various elements; there are electric offset of a read-out circuit and an offset based on dark current which is output from conversion element. The reason why offset correction cannot be sufficiently performed after frame rate switching is that storage time varies depending upon the frame rate. Further, as an element for increasing such an error, there are mentioned gain setting switching of a read-out circuit in conformity with switching of frame rate, and change of the number of lines acquired by pixel addition.
In patent documents 1 and 2 cited above, there is employed an approach to have offset correction data for every switching pattern of frame rate to deal with such problems.